Flexure of the edentulous mandible has been connected to increased stress in splinted full-arch restorations that cross the midline. The increased stress has been reported to result in fracture of abutment screws, fracture of dental implants, fracture of the veneering porcelain and perhaps even pain in the jaw during function.
Mandibular flexure has been evaluated using elastomeric impression materials and finite element analysis. Dental implants have served as a reference point due to the rigid connection with the supporting alveolar bone compared to the movement of natural teeth attributed to the periodontal ligament. Typically, the smaller the mandible in terms of volume, the more flexure. When considering that dental implant-supported restorations were initially considered for mal-adaptive denture patients, it would be expected that flexure of the edentulous jaw would be an important factor to consider.
A review article related to mandibular flexure (Law, et. al. 2012) evaluated 20 articles that met their inclusion criteria. Their conclusion was, “The clinical significance of mandibular flexure on the success of dental implant treatment is at this time unclear, and further research is needed”
There has to be more to it than that … Right?
Of course, there is always another side to the debate. Miyamoto, et. al. (2003) concluded that “jaw flexure is a primary cause of posterior implant loss in full arch mandibular prostheses.” This sounds a little more serious.
Dhima, et. al., (2014) published a retrospective evaluation of more than 250 full-arch fixed hybrid (acrylic/denture teeth) prostheses made between 1983 and 1991. Only 15 percent of the prostheses survived free of any event (repair, removal of an implant, etc.). The conclusion was that this type of restoration should be thought of in a chronic condition context. It is interesting to note that dental implant failure over this time frame was recorded at 9 percent.
Korioth & Hannam (1994) created a model for finite element analysis to evaluate mandibular flexure with five functional positions related to tooth contact. The degree of elastic distortion depends on the specific clenching task – intercuspal position, unilateral group function with and without balancing contacts as well as isolated clenching at either the incisal area or the molar area. Specifically, mandibular flexure has been measured to occur in the body or corpus of the mandible, at the gonial angle, and at the neck of the condyles. Jiang (2002) defined the magnitude of movement related to mandibular flexure as being less than the depressible limit of the periodontal membrane (0.2 mm).
The degree of mandibular flexure may be OK with a splinted full arch tooth supported prosthesis, the 0.2mm measured by Jiang (2002) might be enough to create a problem with dental implant supported prostheses. Likely more of a concern for dental implants positioned distal to the mental foramen as a result of the medial flexure of the body/corpus of the mandible.
Can the stiffness of the connecting prostheses counteract mandibular flexure?
Korioth and Johann (1999) evaluated material characteristics and superstructure design for dental implant-supported prostheses with simulated functional movement. A rectangular beam with a longer vertical component resulted in the lowest measured stress at the level of the abutment. The relative stiffness of the superstructure increases with the vertical component resulting in a decreased stress at the abutment level.
Any guidelines to manage flexure of an edentulous mandible?
Mandibular flexure is measureable in three important functional situations: Clenching, Protrusion and Maximum Opening where fracture of the restorative material and/or pain may be a reported finding.
Three guidelines are:
- Making the final full-arch impression with 20 mm or less of opening to capture the three-dimensional implant position with minimal flexure of the mandible … easier to accomplish with an open tray technique where the dental implants are anterior to the mental foramen.
- Consider planning dental implant position to create smaller fixed bridge units when adding implants distal to the mental foramen to support a full-arch non-splinted fixed restoration
- In terms of prosthesis design, allow enough space in a vertical dimension for all of the biomaterials involved to maximize the stiffness of the dental implant supported fixed and/or removable prosthesis
What are your thoughts on managing mandibular flexure??
Douglas G. Benting, D.D.S., M.S., F.A.C.P., Spear Visiting Faculty
Mandibular Flexure and its Significance on Implant Fixed Prostheses: A Review. Law, et. al. Journal of Prosthodontics 21(3):219-224;2012
Effect of the Additional Installation of Implants in the Posterior Region on the Prognosis of Treatment in the Edentulous Mandibular Jaw. Miyamoto, et. al. Clinical Oral Implants Research 14:727-733;2003.
Practice-Based Evidence from 29-Year Outcome Analysis of Management of the Edentulous Jaw Using Osseointegrated Dental Implants. Dhima, et. al. Journal Of Prosthodontics 23:173-181;2014.
Deformation of the Human Mandible during Simulated Tooth Clenching. Korioth TWP & Hannam AG. Journal of Dental Research 73:56-66;1994
Influence of Mandibular Superstructure shape on Implant Stresses during Simulated Posterior Biting. Korioth TWP & Johann AR. Journal of Prosthetic Dentistry 82:67-72;1999.
In Vivo Mandibular Elastic Deformation during Clenching on Pivots. Jiang T & AI M. Journal of Oral Rehabilitation 29:201-208;2002.